In the following we will define certain terms as they should be understood according to the present description.
The term “security feature” describes an element that can be used for authentication purposes. Such a security feature can be in any form, i.e. an image or a graphic element. It may comprise a serial number, a printed text, a printed pattern, a designs or code made of a security ink, an intaglio printed pattern or design, a security thread or stripe, a window, fibers, planchettes, a foil, a decal, an hologram, microprintings, a 3-D security ribbon, and/or watermarks. Further the security feature as described herein may comprise a pattern representing a code selected from the group comprising special characters, series of alphanumerical characters and combinations thereof. Alternatively, the security feature may comprise a 1-dimensional barcode, a stacked 1-dimensional barcode, a 2-dimensional barcode (such as a DataMatrix or a QR-Code) and/or a 3-dimensional barcode. Such a code may comprise additional or redundant information in an encoded form so that it is generally not readable or understandable without a key or a procedure to decode the encoded information. The security feature may further be invisible to the naked eye.
An “image” according to this description can be an image that is immediately detectable with the naked eye, or can be a latent image, as defined below. An “image” can also comprise one or more areas that is/are immediately detectable by the naked eye, and/or one or more areas forming a latent image. A printed object comprises an image as defined above, and in one embodiment of the present invention the printed object is in the form of a security feature.
“Latent images” according to this description may comprise images comprising hidden patterns, which are not immediately detectable with the naked eye, but become detectable after a suitable physical, mechanical or chemical treatment or illumination. Latent images may be used in security applications as a security feature. Examples are pressure-sensitive or hot stamped labels with a normal (gray or colored) appearance. When viewed via a special filter (such as a polarizer) an additional, normally latent, image appears. Also so called bleeding inks can provide latent images, which appear or disappear only after specific physical, mechanical or chemical conditions are applied to said inks/images.
In the field of printing it is advantageous if the printed pattern shows good adhesion and mechanical properties on different surfaces, in particular on non-porous surfaces like glass, metals, plastics etc.
Furthermore, in certain applications it can be important that the printed codes are not easy to be reproduced or counterfeited. In order to obtain good adhesion and resistance of the printed image/pattern, the following two approaches are usually employed:                Ink solvent swelling of the medium, i.e. a process wherein the dye inside the ink penetrates the substrate        Reticulation (cross-linking/fixation) of the ink induced by radiation, i.e. using an ink that contains reactive components (monomers, photoinitiators, etc.). “Reticulation” in general describes a polymerization, whereby monomers once reacted generate a crosslinked polymeric matrix.        
Methods involving reticulation are employed, for example, in the processes described in the following documents.
U.S. Pat. No. 7,608,388 B2 relates to lithographic printing members imagable using a combination of inkjet and photopolymerization. The lithographic printing members are said to comprise a photosensitive top layer containing a photo-polymerizable moiety and the first component of a two-component photo-polymerization initiating system. This top layer contains acrylates having one or more reactive acrylic moiety which undergo photopolymerization and become crosslinked when reacted with an imaging fluid containing the second component of the two-component photo-polymerization initiating system and subjected to actinic radiation. Removal of the non-image portions of the top layer with a solvent allows a printing member with an imagewise lithographic pattern on it to be obtained.
U.S. Pat. No. 7,632,423 B2 describes reactive fine particles comprising one or more functional compounds such as latent curing agents. It also describes a liquid thermosetting compound comprising the reactive fine particles and adapted to be cured thereby, e.g. through initiation of cross linking and/or polymerization of the thermoset polymer. The liquid thermosetting compound may be used in the formulation of an ink.
U.S. Pat. No. 8,342,669 B2 relates to reactive ink components and methods for forming images using reactive inks. It describes an ink set comprising at least two inks that mix or combine to initiate a free radical polymerization reaction, thereby leading to image formation.
US 2013/0271526 A1 relates to a bicomponent reactive ink for ink jet printing, wherein the first component comprising a polymerizable epoxy monomer, and the second component comprising a polymerization catalyst. It also describes a method to employ the reactive ink comprising the step of separately jetting the two components of a bicomponent reactive ink composition onto a non-porous substrate, thereby promoting the cationic polymerization of the epoxy monomer.
U.S. Pat. No. 7,699,918 B2 relates to reactive ink components and methods for forming images using reactive inks. In particular, it describes a reactive ink set including three mixtures of radically polymerizable monomers. The first mixture includes a peroxide, the second mixture includes a peroxide decomposition agent, and the optional third mixture does not include a peroxide or a peroxide decomposition agent. A ink jetting device for use with the reactive ink set comprises different channels or reservoirs for storing and maintaining separation of the first, second and third inks. The inks are mixed or combined together before or during jetting onto a substrate or on a substrate after jetting, to thereby initiate the radical polymerization resulting in the formation of a hard, solid ink.
U.S. Pat. No. 8,807,697 B2 describes an encapsulated reactive ink and a method for forming images using the same. The ink includes at least one first reactive component, at least one second component comprising a triggerable component, at least one third reactive component, and an optional colorant; wherein the at least one first reactive component and the at least one third reactive component are capable of reacting with one another to form a solid ink on a substrate; wherein the at least one first reactive component is encapsulated in a microcapsule; wherein the ink can be jetted onto a substrate and treated whereby the treatment causes the at least one triggerable component to trigger the rupture of the microcapsule thereby releasing the at least one first reactive component from the microcapsule so that the at least one first reactive component and the at least one third reactive component come into contact, react, and polymerize thereby curing the ink. The rupture of the microcapsule may be triggered via exposure to radiation.
US 2005/0014005 A1 concerns ink-jettable reactive polymer systems for free-form fabrication of solid three-dimensional objects. It describes a method comprising a) ink-jetting a first ink-jettable composition containing a reactive build material and a second ink-jettable composition containing a curing agent separately onto a substrate such that contact between the reactive build material and the curing agent occurs, thereby resulting in a reaction that forms a solidifying composition, and b) repeating the ink-jetting step such that multiple layers of solidifying composition are accrued, wherein said multiple layers are successively bound to one another to form the solid three-dimensional object.
Specific methods to print security features are employed, for example, in the processes described in the following documents.
US 2014/0049034A1 relates to print product for use as spare part of vehicle brake, which has two line structures including two sets of parallel lines that are applied on printing substrate, where structures are printed using ink that includes color pigments with tilt effect.
U.S. Pat. No. 6,245,711B1 discloses a thermosensitive recording material for register receipts and ATM receipts, which has a latent image which forms pseudo watermark and/or comprises pigment or dye with variable light absorption and/or transmission properties.
U.S. Pat. No. 7,845,572B2 discloses a method of incorporating latent image in apparent solid-color background for representing desired solid color background having target color, comprising printing underlying solid-color background and line-screen patterns on print medium.
U.S. Pat. No. 6,306,929B1 discloses a bleeding ink especially for use in the printing of security documents which comprises a dyestuff and solid binder matrix forming compound(s) by polymerization and/or crosslinking, for use in bank cheques or shares.
However, in all of these methods printing and reticulation results in the formation of images wherein polymer fixing the image exhibits properties that are uniform throughout, meaning that they are not suitable for preparing images with hidden patterns (latent images) in a single-pass operation.
These printing systems exhibit the disadvantage that a print on porous and non-porous surfaces with good mechanical properties and adhesion is very difficult to achieve. An image characterized by the presence of areas with different physical, chemical and mechanical properties (hardness, rub resistance, solvent resistance, adhesive tape resistance) is very difficult to realize. Areas with different properties cannot form easily, for example, a latent image.
Furthermore, these printing systems are very cumbersome, slow, expensive, non-flexible, and cannot produce latent images with high resolution, contrast and good control of the physical and chemical properties.
Objective
The object of the invention is to solve the above cited disadvantages exhibited by the present state of the art.
The objective addressed by the invention claimed herewith is, in particular, the provision of a new printing system that allows the incorporation of more than one polymer into the same printed image (pattern, text, graphic, etc.) in a single-pass operation to thereby enable the formation of one or more images usable as a security feature in security applications. In one embodiment, the one or more images comprise a latent image, and in a further embodiment the one or more images are latent images.
Object of the invention is also to provide an inexpensive, compact, rapid, flexible printing system for printing security features capable of printing rapidly with high resolution latent, not immediately optically detectable images or codes, whereby a print on both porous and non-porous surfaces with good mechanical properties and adhesion is achieved, and whereby an image characterized by the presence of areas with different physical, chemical and mechanical properties (hardness, rub resistance, solvent resistance, adhesive tape resistance etc.) can be easily and rapidly obtained.